In the processing of fiber suspensions, as a rule, several such pressure sorters are connected one behind the other, with the screen openings of the pressure sorters connected one behind the other becoming smaller and smaller, i.e., the screen openings of the pressure sorters following a first pressure sorter are always smaller than the screen openings of the preceding pressure sorter. In this way, impurities and clumps of fibers are separated in dependence upon their size stepwise from the usable fibers as the so-called accepted material of the preceding pressure sorter is fed to each pressure sorter (the accepted material is that part of the fiber suspension which has passed through the screen openings of a pressure sorter).
The paper industry demands apparatus with greater and greater capacity; for pressure sorters this means that not only a higher and higher specific throughput capacity is called for (amount of fiber suspension passing through a screen surface of a certain size per time unit) but that a certain sorting fineness is to be attainable with fewer and fewer pressure sorters connected one behind the other, which is only achievable with relatively fine screen openings, which does, however, conflict with the increasing of the throughput capacity.
Therefore, to increase the throughput capacity not only new rotor shapes were developed, but the circumferential speed of the rotor regions adjacent to the inlet side of the screen was also increased in order to generate relatively high positive and negative pressure thrusts and turbulences in the fiber suspension - clogging of the screen openings by impurities contained in the fiber suspension is to be prevented by the pressure thrusts (backwashing effects occur at the screen openings) and the turbulences are to prevent the fibers from forming at the inlet side of the screen a kind of fibrous mat which reduces the throughput capacity or prevents passage of usable fibers through the screen openings altogether. The measures for increasing the throughput capacity (increasing the rotor speed and the rotor circumferential speed as well as increasing the pressure difference occurring at the screen) do, however, result in the forces acting on the screen becoming greater and greater, and they often cause rapid destruction of the screen, in particular, as a result of cracks occurring in the screen plate. To achieve higher service lives, i.e., to obtain more resistant screens, one has, therefore, increased the wall thickness of the screens; as a result of this measure, the flow channels formed by the screen openings are, however, also increased in length, a consequence which negatively affects the throughput capacity of the pressure sorter. Moreover, the demand for more and more efficient apparatus has not only resulted in a pressure sorter having to process greater and greater fiber suspension quantities per hour, but in it also having to process fiber suspensions with a higher substance density (fiber component per volume unit) and, above all, with fiber suspensions of relatively high substance density, comparatively long screen opening channels lead to high pressure losses between the inlet side and the outlet side of the screen which, in turn, result in rapid clogging of the screen openings.
A wide variety of suggestions has already been made for the design of the screen wall of such screens:
In U.S. Pat. No. 3,581,903 of the applicant, it was suggested that in a screen with slit-shaped screen opening channels a boat-shaped recess be milled in the outlet side of the screen wall for each screen opening and such a screen be installed in a pressure sorter in which the rotor rotates adjacent to the outlet side of the screen. This measure does considerably reduce the danger of clogging of the screen openings by impurities contained in the fiber suspension, but with such pressure sorters there is the danger that a fibrous mat or the like will form in the fiber suspension to be sorted on the inlet side of the screen and drastically reduce the throughput capacity.
A screen for pressure sorters is proposed in DE-AS 27 50 499. This is comprised of ring-shaped segments stacked one on top of the other in the direction of the screen axis and each comprising a ring of slit-shaped screen opening channels, each two adjacent segments defining rings of recesses which extend from the inlet side and the outlet side of the screen as far as the screen opening channels and taper towards the latter. Such a screen does possess relatively short screen opening channels in the flow-through direction and it is extremely resistant to high pressure thrusts generated by a rotor, but it is highly elaborate in its manufacture and cannot prevent particularly effectively formation of a fibrous mat on the inlet side of the screen.
The same is applicable to the screen disclosed in EP-O 093 187-B of the applicant; this is provided on the inlet side with grooves extending around the entire screen in the circumferential direction of the screen and on the screen outlet side with milled, circular-cylindrical recesses, the centers of which lie in the points of intersection of a net formed by equilateral triangles and the bottom surfaces of which are perforated by the bottom of the grooves. The purpose of this screen configuration is essentially the same as in the screen according to the previously discussed U.S. Pat. No. 3,581,903, i.e., the flow-through channels formed by the screen openings are widened in the flow-through direction by the circular-cylindrical recesses. On the other hand, with this screen configuration, too, formation of a fibrous mat on the inlet side cannot be prevented particularly effectively.
From EP-0042742-B there is known a screen for pressure sorters comprising a rotor adjacent to an inlet side of the screen. The screen is of rotationally symmetrical design in relation to a screen axis, and the screen wall which is integral throughout its wall thickness is provided with screen opening channels connecting the screen inlet side with the screen outlet side and on its inlet side with recesses into which the screen opening channels open. In a first embodiment of the known screen, the screen opening channels are in the form of slits extending transversely t the circumferential direction of the screen and the recesses on the inlet side are boat-shaped recesses which are milled into the screen plate and likewise extend transversely to the circumferential direction of the screen, one such boat-shaped recess being provided for each screen opening channel (FIGS. 2 to 4 of EP-0042742-B). In a second embodiment of this known screen, the screen opening channels are circular-cylindrical bores, each having associated with it a frustoconical recess which opens into the inlet side of the screen and widens towards it with a relatively small aperture angle (FIGS. 5 and 6). With the recesses on the inlet side an increase in the throughput capacity of the pressure sorter is to be achieved by these recesses on the inlet side causing together with the rotor rotating on the inlet side turbulences which counteract the formation of a fibrous mat on the inlet side of the screen. However, an unsatisfactory throughput capacity is to be ascertained with both embodiments of the known pressure sorter, quite aside from the fact that it involves considerable expenditure to mill not only the individual slit-shaped screen opening channels but also a boat-shaped recess for each individual screen opening channel or in the second embodiment to make a frustoconical recess for each individual screen opening channel on the screen inlet side.
Finally, a pressure sorter with a circular-cylindrical screen is known from WO 87/03024. Herein screen opening channels in the form of circular-cylindrical bores connecting the screen inlet side with the screen outlet side open into groove-shaped, parallel recesses extending transversely to the circumferential direction of the screen and provided on the screen inlet side and the screen outlet side, whereby the efficiency of the screen is to be improved. This measure does result in a reduction of the pressure loss at the screen because the narrow screen opening channels--relative to the screen wall thickness--are relatively short, but the known screen design has a serious disadvantage: the groove-shaped recesses on the inlet and outlet sides extending transversely to the circumferential direction of the screen result in the formation of weakening lines in the screen wall extending parallel to the screen cylinder axis and hence to the generatrices of the screen cylinder so that it is not only problematic to produce a screen cylinder which is absolutely symmetrical with respect to rotation from a finished screen plate by bending, but, in addition, these weakening lines involve the risk of permanent fractures in the screen wall owing to the pressure thrusts generated by a rotor.